Despite extensive research efforts during the past decades, there is no general agreement about pathophysiology, or reliable biomarkers for irritable bowel syndrome (IBS). This lack of progress is reflected in the continued excessive direct and indirect health care expenditure related to these conditions, largely in form of unnecessary diagnostic procedures to rule out organic disease and lack of novel, cost effective therapies. During the past funding cycle, we have identified functional alterations of brain networks in IBS patients, both in the resting state and in response to physical and psychological stimuli, which are related to sex, gene polymorphisms and a history of early adverse life events. These functional findings were accompanied by extensive structural remodeling of both white and grey matter in the brain of IBS patients resulting in a distinct brai signature. In this revised competitive renewal application, I plan to take these insights to the next level by validating the accuracy of structural/functional brain signatures to differentiate IB patients from healthy and from disease control subjects, and by identifying associations of brain signatures with changes in the peripheral immune system and the gut microbiome. Using an array of advanced, innovative brain imaging, genomic and microbiological techniques, I will address these goals in 3 Specific Aims: A. Identify disease- specific brain signatures of IBS, identify correlations of these signatures with behavioral and clinical parameters, and determine accuracy to predict IBS phenotype compared to HCs, and patients with chronic gut inflammation (ulcerative colitis). B. Identify gene expression profiles in peripheral blood monocytic cells (PBMCs) and correlate with distinct brain signatures in IBS patients as identified in Aim A. C. Identify gut microbiome derived metabolites which correlate with brain signatures in IBS patients. The longitudinal design of the study combined with a mediator/moderator analysis approach will enable us to infer causality between brain, peripheral factors and symptoms, and will provide the basis for future mechanistic studies in animal models. The long-term goal of the proposed studies is to improve our understanding of the complex bidirectional brain gut interactions of the brain gut microbiome axis in IBS and identify biomarkers in order to develop more effective treatment strategies.